1. Field of the Invention
The present invention relates to a method for forming metal clusters, and more particularly, the present invention relates to a method for controlling the size and density of catalytic clusters formed on a surface and a substrate having metal cluster formed on a surface thereof.
2. Background of the Invention
Metal clusters with diameters in the range of 1-10 nanometers (“nm”) are extremely effective catalysts given that nearly all of the metal atoms comprising the cluster are present on the surface and therefore available to participate in catalytic reactions.
Metal clusters are difficult to prepare in mono-dispersed form having a narrow size distribution. This is particularly difficult when attempting to deposit the clusters on high surface area substrates which are necessary to support heterogeneous catalysts.
A traditional method for preparing mono-dispersed metal clusters is to use a molecular beam in which ionized metal clusters are size-selected using a quadrupole or electrostatic filtering element.
Metal clusters are also formed by evaporating or sputtering a thin metal film onto a surface and annealing the film such that the metal atoms coalesce to form particles or clusters. Both of the aforementioned techniques are only viable for deposition on flat (i.e., nonporous) surfaces.
Metal nanoparticles may also be created using solution based methods. However, the nanoparticles must still be dispersed on the high surface area support without agglomeration to create the heterogeneous catalyst. Moreover, metal nanoparticles fabricated in solution are typically coated with one or more mono-layers of stabilizing agents, such as polymers, to prevent aggregation of the metal particles, and these layers must be removed in order to use the particles as a catalyst.
Porous supports can be infiltrated with metal atoms using wet chemical or chemical vapor techniques followed by annealing, causing the atoms to aggregate into clusters. However, this method offers little control over particle size and density.
Atomic layer deposition (“ALD”) produces conformal films of a variety of materials including some metals with exquisite thickness control by utilizing alternating reactions between gaseous precursor molecules and a solid surface to deposit material in a layer-by-layer fashion. However, attempts to deposit metal on oxide foundation substrates using ALD has met with limited success. This is due to the lack of metalorganic chemisorption on oxide terminated surfaces. J. Senkevich, et al, Chem. Vapor Depos. 8 (2002) 189.
A need exists in the art for a method for depositing catalytic clusters on porous surfaces, and particularly oxide surfaces. The method should enable the formation of clusters between 1 nm and 10 nm in diameter. The method should further enable the formation of clusters in a variety of cluster densities without increasing the size of the individual clusters. Conversely, the method should enable the formation of large clusters without a concomitant increase in the number of clusters.